class Node:
    
    def __init__(self,value):
        self.value = value#当前节点
        self.next = None#下一个节点

class List:
    def __init__(self):
        #头节点
        self.head = Node(-1)
#前插法创建单链表
    def insert_before(self,data):
        for i in data:
            node = Node(i)#创建新节点
            #判断头节点的下一个节点是否为空
            if self.head.next is None:#如果为空，则将新节点加入到next
                self.head.next = node
            else:
                node.next = self.head.next#将头节点的下一个节点加入到当前节点的next
                self.head.next = node
#尾插法创建单链表
    def insert_tail(self,data):
        tail = self.head.next
        for i in data:
            node = Node(i)#创建新节点
#判断下一节点是否为空
            if tail is None:#如果为空
                self.head.next = node
                tail = node
            else:
                tail.next = node
                tail = node
#删除链表中重复元素
    def clear_repetiton(self):
        cur=self.head
        while cur:
            while cur.next and cur.value==cur.next.value:
                cur.next=cur.next.next#删除重复元素
            cur=cur.next
#第i个节点前插入
def list_element_add(self,i,value):
    node_new = Node(value)#创建新节点
    index = 0
    node = self.head.next
    while node:#找位置
        index = index + 1  
        if index == i - 1:
            break
        node = node.next
    if node is None:
        return False
    node_new.next = node.next#插入节点
    node.next = node_new
